Process for the preparation of colored polymer powders of controlled charge and printing characteristics

ABSTRACT

A process is provided for preparing colored polymer powders with controlled charge and printing characteristics when used as toners in electrostatic printing. The process comprises a controlled heating, melting, and dispersion of a polymer in the presence of water and surfactant. The polymer contains a coloring agent and a specially selected charge-directing agent, such as a nigrosine dye or a metal powder. Colored powders can thus be obtained composed of spherical particles which have exceptionally fine printing characteristics, when used as toners in electrostatic printing processes, and which have controlled average size and size distribution.

United States Patent Bartsch et al.

[ 51 June 13, 1972 [54] PROCESS FOR THE PREPARATION OF COLORED POLYMERPOWDERS OF CONTROLLED CHARGE AND PRINTING CHARACTERISTICS [72]Inventors: Raymond C. Bartsch; Frank Lerman, both of Cincinnati, Ohio[73] Assignee: National Distillers and Chemical Corporalion, New York,NY.

[22] Filed: May 21, 1970 [21] Appl. No.: 39,513

[52] US. Cl. ..260/41 R, 96/1 SD, 106/308 Q,

1l7/17.5, 252/62.l, 260/2.5 R, 260/29.6 R, 260/34.2, 260/41 B, 260/41 C,264/5, 264/9,

264/15, 264/117 [51] Int. Cl ..C08f 1/86, G03g 9/02 [58] Field of Search..106/308 Q; 260/41 R, 41 B,

260/41 C, 34.2, 29.6 R, 2.5; 264/5, 9, 15, 117; 252/62.1; 96/1 SD;l17/17.5

[5 6] References Cited UNITED STATES PATENTS 3,244,633 4/1966 Yellin etal. ..252/62.1

3,449,291 6/1969 Lerman et a1 ..260/41 FOREIGN PATENTS OR APPLICATIONSOTHER PUBLICATIONS lntemational Encyclopedia of Chemical Science (VanNostrand) (N.Y.)(l964),page 772.

Bickel et al. ..260/33.2 R Eastes et a1.

Martins Palermiti et al. ..252/62.1

Belgium Primary Examiner-Morris Liebman Assistant Examiner-H. H.Fletcher Attorney-Allen A. Meyer, Jr.

[5 7] ABSTRACT 18 Claims, No Drawings PROCESS FOR THE PREPARATION OFCOLORED POLYMER POWDERS OF CONTROLLED CHARGE AND PRINTINGCHARACTERISTICS The availability of polymer powders in recent years hasdeveloped a number of industrial uses, and as the uses have become morehighly refined, demand has arisen for colored powders consisting ofuniform particles, preferably spherical in shape, with a controlled sizeand size distribution and certain other specific physical properties.Polymer powders are used to coat various types of articles, bydip-coating in a stationary or a fluidized bed of the powder, by powdercoating (wherein the powder is applied by spraying or dusting), by flamespraying, and by electrostatic attraction. In these uses, the polymerpowders are necessarily thermoplastic or in a thermoplastic stage ofpolymerization, since the melting of the powders is a necessary step inthe adhesion of the particles to the base, and in the formation of acontinuous coating film. Such polymer powders have also been applied indispersed form as coatings by roller coating, spray coating, slushcoating, dip coating, and electrostatic coating, to substrates such asmetal, paper, paperboard, and the like.

These powders have also been employed in conventional powder moldingtechniques; as additives to waxes and paints and polishes; and asbinders for nonwoven fabrics.

Electrostatic copying, duplicating, printing and gravure processes haveopened new requirements for powders consisting of black, white orcolored particles of narrow size distribution and controlled size, andother physical, chemical, mechanical and electrostatic properties, foruse as toners or inks in the dry form or suspended in liquid, and asdevelopers for electrostatic coating processes, such as in the Xerox andElectrofax copying processes.

In these uses, it has become increasingly important that the particlesof the polymer powder have consistent and stable properties, and beavailable in a controlled size and size distribution. Particularlyimportant, when used as toner, is the charge acquired by the particles,its intensity and the charge decay rate. The particles should acquire apositive charge, when used in the Electrofax dry-toner copying process,and a negative charge when used in the Xerox process, and this chargehas to be of a sufficient intensity to form a dark print having sharpdefinition of the image. The particles must also be colored, for goodcontrast with the background of the base on which the print is laiddown.

Such properties are particularly desirable in specialized researchstudies, using the particles as aerosol tracers, as simulants, and asstandards for study of chemical, biological, meterological, andradioactive air dissemination, and for oceanography studies. It istherefore important to be able to prepare these materials by a processthat is easy to carry out, and that is precisely duplicatable, so as toproduce particles of controlled and standardized properties through ajudicious selection of the polymeric material, additives, processingmedia, and process operating conditions.

Colored thermoplastic powders can be made by grinding coarse, coloredthermoplastic cubes, pellets, etc., to the desired sizes. Such products,in varying particle sizes, can be made by incorporating pigments or dyesin a mixture or blend of resins which are subsequently passed through ahigh shear pulverizing device, and then size-classified on a shakerscreen or in an air classifier. The particles thus produced areirregular and nonuniform in shape and the particles should be uniform inshape, since a uniform spherical shape contributes superior flow andfluidization characteristics, and improved dispersibility to thepowders.

The surface of polymer particles can be coated with a coloring agent byconventional dyeing techniques. However, dyeing particles by coatingthem on the outside is a difficult procedure, and unless special care isexercised, the coating is nonuniformly distributed, and the material istacky. Moreover, surface coatings can be removed by natural friction andabrasion during powder flow, or by solvents. Where color is added to theparticles, uniformity of color distribution among and within theparticles is important for overall uniform color effects of the powder,and/ or of the products or coatings formed from it, or in particledetection, and in quantitative determinations in air dispersion andtracer studies.

In some uses, it is important for the particles to have densitiesdifferent from the polymer density. Density can be decreased byincorporation of foaming agents, or increased by incorporation offillers. The polymer properties can be modified by incorporation ofother polymer additives, and here also it is important that theadditives be uniformly distributed in the polymer, so that theindividual particles in addition to being of a uniform size and shape,will be uniform in the desired property.

The prior processes for preparing polymer powders from coarser forms,such as cubes, pellets, chips, flakes, granules, and the like, whichforms usually are available commercially, are of three main types: (I)mechanical grinding to form coarse or fine powders, and (2) solution,and (3) dispersion, to form fine powders.

Coarse polymer powders are obtained by mechanically grinding the coarserforms by passing them through a high shear pulverizing device, such as aPallmann grinder, to yield particles of irregular shape, havingdiameters ranging from about 75 to about 300 microns. Such powders arenot suitable for many applications, where spherical particles of thesame, slightly larger or much finer size, sometimes desirably in anarrow size distribution, are necessary.

The ground powders are classified as to particle size on a shaker screenor in an air classifier. Grinding and size-classifying are expensiveprocedures, requiring excessive power, close control, and special andexpensive equipment. Moreover, an appreciable part of the classifiedmaterial is off-size, and must be reprocessed, used for other purposes,or disposed as waste. In addition, since the particles produced areirregular and nonuniform in shape, they are not entirely suitable formany applications, wherein spherical particles are preferred.

In the solution process, the polymer is dissolved in a solvent, and thenprecipitated from the solvent in finely divided form. The precipitationis accomplished by addition of a nonsolvent which is miscible with thesolvent, and therefore rapidly reduces solubility of the polymer in thesolvent; or by evaporation of the solvent to exceed the solubility ofthe polymer; or by a combination of the two methods. Emulsifying agentscan be used, to aid in breaking down the size of the particles formed bysuch precipitation techniques. In these processes, there aredifficulties in handling the solvent, and in completely removing thesolvent from the polymer particles. Also, the resulting particles are ina wide size distribution, and must be classified, if particles of anarrow size distribution are desired. Also, the particles from theseprocesses are of an irregular although somewhat rounded shape. Thisprocessing is costly, and not entirely satisfactory for manyapplications.

The dispersion process requires the suspension of the polymer in aliquid medium, with the aid of dispersing agents, after which thedispersion is subjected to high shear agitation. Water is generally thepreferred liquid medium, because of low cost and simplicity ofoperation. The usual dispersing agents are soaps, such as sodiumstearate. The polymer is then reduced to a molten condition, and theresulting molten mass is then dispersed in the liquid medium. If thedispersing agent is incorporated in the polymer before dispersion, thepresence of the dispersing agent residues in the polymer may createundesirable changes in the polymer properties, for example, increasedwater sensitivity, reduced electrical resistivity, and otherdifficulties. Removal of these residues is, however, difficult, if notimpossible. A further difiiculty is that such dispersing agents tend tobecome inactive at elevated temperatures, as a result of which theoperating temperature range is so low that only relatively low molecularweight polymers, such as low molecular weight polyethylene, aresufficiently fluid at such temperatures to be dispersible in water.These processes have therefore not been applicable to the preferred highmelting thermoplastic polymer types. Also no control of particlecharacteristics are indicated.

In accordance with the present invention, a process is provided forpreparing colored polymer powders of controlled charge-acquiring andprinting characteristics, when used as toners in electrostatic printingprocesses, as well as controlled particle size and size distribution,having consistent and stable physical, chemical, mechanical,electrostatic and aerodynamic properties. By judicious selection of thecharge-directing additive, polymeric material, coloring agent, and anyother additives; processing media; and operating conditions; it ispossible to modify, control and standardize the charge-acquiring andother properties of the colored powder, and the particles of which it iscomposed. Due to such control of the process, it is often quiteunnecessary to subsequently classify these colored powders for thedesired average particle size and size distribution.

The charge acquired by the powder particles of this invention when usedas a toner in electrostatic printing is controlled by thecharge-directing agent. It has been determined, for example, thatcertain dyes are capable of modifying the electrical properties of thecolored particles, so that the powders acquire a positive or negativecharge, as required by the electrostatic printing process in which theyare to be used. Metal particles are capable of further modifying theseproperties, so that an intensified charge, either positive or negativeis acquired by the colored particles. Both such dyes and metal powdersare encompassed by the term charge-directing agent", as used herein.

The charge-directing agent for use in the practice of this invention maybe any suitable dye. It must be heat-stable at the dispersiontemperature of the polymeric material, should not react chemically withthe polymeric material, and preferably should exhibit neglibible ormoderate solubility in water. Moreover, the charge-directing agentshould be light stable and should not bleed or migrate" from thethermoplastic resin after dispersion. Examples of possible materialsinclude azine dyes such as the indulines, nigrosines, and wool fastblues, oxazine dyes such as Meldola Blue and Nile Blue, thiazine dyessuch as methylene blue, azo dyes such as naphthal blue black, sulphoncyanine black, diazo dyes such as Diamond Black F, cyanine dyes,indigosol dyes such as Algosol Blue, anthiaquinone dyes, ketoniminedyes, Xanthine dyes, and indamine and indophenol dyes.

These dyes are not normally thought of as suitable for coloringplastics, and in this invention they are used not as coloring agents butin conjunction with the usual coloring agents, such as dyes andpigments.

Metal particles as a class can be used to intensify the charge-acquired.Pigment-type powders or submicron type powders can be used. Copper,zinc, nickel, iron, cadmium, tin, aluminum, bronze, brass and steelparticles can be used.

Quite small amounts of charge-directing agent are sufficient. As littleas 0.001 percent by weight of the polymer is effective, and amounts ashigh as 25 percent by weight have been used without deleterious effect.Optimum results are obtained at from 0.01 to percent, and such amountsare there fore preferred.

The charge-directing agent and the coloring agent can be uniformlydistributed throughout the particles by compounding or preblending thebulk or particulate polymer therewith, giving a uniform color andcharge-acquiring effect which is stable and durable, because the coloris distributed throughout the mass. The particles have a spherical orother regular shape, contributing superior powder flow and fluidizationcharacteristics, a shorter melting time, improved dispersibility, andless variation in powder and particle characteristics, due to controlledparticle shape, size and size distribution.

In the process of this invention, a polymer (in particulate form or as amolten mass), containing a coloring agent, and a charge-directing agent,is dispersed in an inert liquid by high shear agitation at a temperatureabove the melting temperature of the polymer, in the presence of asurfactant. Agitation is continued at such temperature for a timesufficient to shape the colored particles, and maintain or change theirsize and/ or size distribution. The resulting particle size may be thesame as or larger or smaller than the starting particle size; and theparticle size distribution can also be narrowed or broadened. Thecolored particles are then cooled, so as to solidify them and stabilizethem in that shape and size.

The coloring agent can be any suitable pigment, dye, opacitier,brighteners, and fluorescent agent, or the like. It must be heat-stableat the dispersion temperature of the polymeric material, should notreact chemically with the polymeric material, and preferably shouldexhibit negligible or moderate solubility in water. Moreover, thecolorant should preferably be light-stable and should not "bleed" or"migrate from the thermoplastic resin after dispersion. Examples ofsuitable materials include carbon black, phthalocyanine blue,fluorescent coloring agents or dyes, phthalocyanine green, cadmiumsulfide, cadmium sulfide selenide, titanium dioxide, calcined ironoxide, chromic oxide, zinc oxide, and the like.

The concentration of coloring agent can be within the range from about0.001 to about 1 part by weight per part of the polymer colorantmixture. Usually, the concentration is within the range from about 0.005to about 0.2 part, with the preferred proportion being from about 0.002to about 0.15 part by weight.

In the preferred embodiment of the practice of this invention, thecoloring agent and charge-directing additive are incorporated into thepolymeric material by combining said colorant with a finely divided orgranulated polymer in a mix ing device such as a twin-cone blender andthen mixing the two on a twin-roll mill, in a Banbury mixer, in aconventional screw extruder, or in other suitable equipment where theheat, either applied or generated by friction, is controlled to aid theblending of the agent into the polymer and at the same time to minimizepolymer degradation. The blended material is then granulated orpelletized by conventional means.

The blend is subsequently contacted with water and the selecteddispersing agent in any suitable dispersing apparatus. After heating andagitation, the system is cooled rapidly and the product is recovered,for example by filtration, washing, and drying.

Because of the large number of variables that can be controlled toafiect the shape, size and size distribution of the par ticles, theprocess of the invention is of extraordinary versatility. For any giventype of resin, it is possible to adjust the operating parameters so asto produce particles in a regular shape and surface configuration and inany size within the range from 1 to 1,000 microns, controlled within awide, narrow, or very narrow size distribution. This is accomplished byselection of the appropriate group of process variables, andstandardizing the process operating conditions to obtain the desiredsize and shape of particles. The very great versatility of the processat the same time introduces an element of uncertainty in the predictionof the effect of a given set of variables on a particular polymerwithout trial and experiment, simply because mathematical andphysico-chemical computations are inadequate to accommodate thesevariables in a set of mathematical formulae or equations. The physicalphenomena are extremely complex, involving shear forces, surface tensionforces, densities, viscosities van der Waals forces, and cohesion andadhesion of soft particles, under con ditions virtually impossible tomeasure or even evaluate. It is therefore necessary to establish theproper operating parameters for any given type of particle and type ofpolymer by trial and error experimentation. Such tests afford littledifficulty, however, to one skilled in this art, and are easily carriedout, by taking into consideration the variables that affect particlesize and size distribution.

The thermoplastic material suitable for preparing polymer powders ofthis invention must have negligible or only slight solubility in wateror other liquid medium used, and it must be capable of forming intimate,uniform mixtures with the coloring agent and charge-directing agent. ltis also important that the polymeric material be immiscible in theliquid suspending medium at the operating temperature. Although thisinvention will be described with reference to polyethylene, it is to beunderstood that the scope of the invention includes other olefinhomopolymers and copolymers, such as polypropylene, polyisobutylene, andpolyisopentylene; polyfluoroolefins, such as polytetrafluoroethylene andpolytrifluorochloroethylene; polyamides, such as polyhexamethyleneadipamide, polyhexamethylene sebacamide, and polycaprolactam; acrylicresins, such as polymethylmethacrylate, polyacrylonitrile,polymethylacrylate, polyethylmethacrylate, andstyrene-methylmethacrylate; ethylenemethyl acrylate copolymers,ethylene-ethyl acrylate copolymers, ethylene-ethyl methacrylatecopolymers, polystyrene, cellulose derivatives, such as celluloseacetate, cellulose acetate butyrate, cellulose propionate, celluloseacetate propionate, and ethyl cellulose; polyesters, such aspolycarbonates; polyvinyl resins, such as polyvinyl chloride, copolymersof vinyl chloride and vinyl acetate, and polyvinyl butyral, polyvinylalcohol, polyvinyl acetal, ethylene-vinyl acetate copolymers,ethylene-vinyl alcohol copolymers, and ethylene-allyl copolymers, suchas ethylene-allyl alcohol copolymers, ethylene-allyl acetate copolymers,ethylene-allyl acetone copolymers, ethylene-allyl benzene copolymers,ethylene-allyl ether copolymers, and ethylene-acrylic copolymers; andpolyoxymethylene.

Exemplary thermosetting materials in a thermoplastic stage ofpolymerization are phenol-formaldehyde, urea-formaldehyde,melamine-formaldehyde and alkyd resins, and polyesters.

The surfactants are nonionic, and the preferred surfactants have awater-insoluble nucleus of a polyoxyalkylene glycol other than ethyleneglycol, with a molecular weight of more than 900, which has beenextended with water-soluble polyoxyethylene groups at each end. Thewater-soluble portion of the molecule should constitute at least 50percent by weight of the total. The polyoxyalkylene glycol can bealiphatic, aromatic or alicyclic in nature, can be saturated orunsaturated, and can be represented by the formula:

2 4 )y( m n ),r( 2 4 1/ wherein x, y, m and n are integers. When(C,,,H,,O), is saturated aliphatic, n 2m.

Compounds in this class are described in US. Pat. No. 2,674,619 toLundsted, dated Apr. 6, 1954 and No. 2,677,700 to Jackson et al., datedMay 4, 1954.

The polyoxyalkylene compounds of US. Pat. No. 2,674,619 are defined bythe formula:

YI: a 6 .r where Y is the residue of an organic compound containingtherein x active hydrogen atoms,

n is an integer,

x is an integer greater than 1.

The values ofn and x are such that the molecular weight of the compound,exclusive of E, is at least 900, as determined by hydroxyl number,

E is a polyoxyalkylene chain wherein the oxygen/carbon atom ratio is atleast 0.5, and E constitutes at least 50 percent by weight ofthecompound.

The polyoxyalkylene compounds of US. Pat. No. 2,677,700

are defined by the formula:

wherein:

Y is the residue of an organic compound containing therein a singlehydrogen atom capable of reacting with a 1,2-alkylene oxide, R R R andR, are selected from the group consisting of H, aliphatic radicals andaromatic radicals, at least one such substituent being a radical otherthan hydrogen, n is greater than 6.4 as determined by hydroxyl numberand X is a water-solubilizing group which is nonionic and constitutes atleast 50 percent by weight of the total compound.

The compounds of US. Pat. No. 2,674,619 are available commercially underthe trademark Pluronic." The following are examples of compoundscorresponding to the above formula:

water-soluble nucleus with a molecular weight of at least 900 containingan organic compound having a plurality of reactive hydrogen atomscondensed with an alkylene oxide other than ethylene oxide and havingwater-soluble polyoxyethylene groups attached to each end. The weightper cent of the hydrophilic portion of the molecule should be at least50. This type of emulsifier is available commercially under thetrademark Tetronic." These are ethylene oxide adducts of an aliphaticdiamine such as ethylene diamine extended with propylene oxide havingthe following formula:

where x and y are as defined above.

Compounds in this class are described in US. Pat. Nos. 2,674,619 and3,250,719 and are available commercially under the trademark Tetronic.The following are examples of compounds corresponding to the aboveformula:

Molecular weight Ethylene oxide Molecular for ethylene dicontent infinal weight of amine-propylene product, weight final Name oxide basepercent product Tetronic 707 3,000 75 12,000 Tetronic 908 4,050 27,000

Other compounds in this class include ethylene oxide adducts ofpolyhydroxy alcohols extended with alkylene oxide, ethylene oxideadducts of polyoxyalkylene esters of polybasic acids, ethylene oxideadducts of polyoxyalkylene-extended amides of polybasic acids, ethyleneoxide adducts of polyoxyalkylene extended alkyl, alkenyl and alkynylaminoalkanols, of which the hydrophobic nucleus should have a molecularweight of at least 900 and the hydrophilic part of the molecule shouldbe at least 50 percent by weight of the total. It is to be understoodthat the above-mentioned organic compounds having a plurality of activehydrogen atoms as well as the polyoxyalkylene glycols can be aliphatic,aromatic or alicyclic in nature and can contain unsaturation.

Such compounds can be of the following formulae (m, n, x and y are asabove):

2 4 )y m n )x( 1 n 1 14 m n )x 2H4 h amine dlalcohol (could be tri-) Athird group of nonionic emulsifiers that can be employed includes highmolecular weight polyoxyethylene adducts of hydrophobic organiccompounds having one active hydrogen, such as aliphatic, saturated orunsaturated alcohols having at least 18 carbon atoms; monoordi-substituted alkyl, alkenyl or alkynyl aromatic or alicyclic alcoholsof at least 15 carbon atoms; monobasic aliphatic, saturated orunsaturated aromatic or alicyclic monobasic hydroxy acid derivativessuch as N-alkyl, -alkenyl or -alkynyl amides or alkyl, alkenyl oralkynyl esters of at least 18 carbon atoms; alkyl, alkenyl or alkynylglycol monobasic acid esters of at least 18 carbon atoms; or di-N-alkyl,-alkenyl or -alkynyl (aromatic or alicyclic) aminoalkanols having atleast 18 carbon atoms. The hydrophilic portion of these molecules shouldbe at least 50 percent by weight of the total. Such compounds can havethe following formulae (m, n, x and y are as above):

propylene oxide and ethylene oxide, molecular weight 4,500.

In the preferred embodiment, the coloring agent and charge-directingagent are incorporated into the polymeric material by combining themwith a finely divided or granulated polymer in a mixing device such as atwin-cone blender and then mixing the two on a twin-roll mill, in aBanbury mixer, in a conventional screw extruder, or in other suitableequipment where the heat, either applied or generated by friction, iscontrolled to aid the blending of the agents into the polymer and at thesame time to minimize polymer degradation. The blended material is thengranulated or pelletized by conventional means.

The blend is subsequently contacted with water and the selecteddispersing agent in any suitable dispersing apparatus. After heating andagitation, the system is cooled rapidly and the product is recovered,for example, by filtration, washing, and drying.

The dispersing apparatus may be any device capable of delivering atleast a moderate amount of shearing action to a liquid mixture underelevated temperatures and pressuresv An example of suitable apparatus isa conventional autoclave equipped with conventional propeller stirrers.Propellers designed to impart greater shear to the mixture can affectthe average particle size and size distribution of recovered polymer.The average particle size and size distribution of the CIBHZWO (C 11O)yII (llsHn Alcohols.

Amine.

Disubstituted phenol.

Amide of hydroxy acid.

Ester of hydroxy acid.

Ester of monobasic acid.

Additional suitable surfactants are water-soluble block powder productare influenced by the type of equipment,

copolymers of ethylene oxide and propylene oxide. They are preferablywater-soluble block copolymers of ethylene oxide and propylene oxidehaving a molecular weight above about 3,500 and containing a majorproportion by weight of ethylene oxide. Such compounds are both stableand effective as dispersing agents for thermoplastic polymers attemperatures ranging up to about 500 F. or higher, and more particularlyat temperatures above about 280 F., especially temperatures in the rangeof about 280 to 400 F. Representative of such compounds are thoseprepared by polymerizing ethylene oxide on the ends of a preformedpolymeric base of polyoxypropylene. Both the length and the molecularweight of the polyoxypropylene base and the polyoxyethylene and segmentscan be varied to yield a wide range of products. One example of asuitable surfactant is a polyoxypropylene of average molecular weight of2,700 polymerized weight ethylene oxide to give a product of molecularweight averaging about 13,500; it contains about 20 weight percent ofpropylene oxide and about 80 weight percent of ethylene oxide. Otherefiective agents include (a) 50 weight percent each of propylene oxideand ethylene oxide, molecular weight 6,500; (b) 20 weight percent ofpropylene oxide and 80 weight percent of ethylene oxide, molecularweight 11,250; (c) 20 weight percent of propylene oxide and 80 weightpercent of ethylene oxide, molecular weight 16,250; and (d) 50 weightpercent each of agitation time, stirring rate, and other operating anddesign factors. Higher stirring speeds generally result in finer andnarrower dispersions until an optimum speed is reached. The stirringperiod at dispersion temperatures is generally from about 1 to 24minutes at agitator tip speeds of from 400 to 4,000 linear feet perminute, but preferably about 5 to 15 minutes at tip speeds of about 600to 1,000 linear feet per minute. The stirring rates and periods,however, depend upon the material processed and the type of equipmentused.

The surfactant need not be incorporated by milling or the like into thepolymer in advance but may be introduced into the dispersing apparatusas a solid or as an aqueous solution simultaneously with the otheringredients. If desired, the dispersing process may be operated in acontinuous manner.

The operating conditions, such as polymer concentration, ratio ofsurfactant to polymer, agitation tip-speed, agitation time, andoperating temperature depend upon the type of polymer used, the type andconcentration of the coloring agent and charge-directing agent. Thesurfactant type, the liquid medium selected, and the particle sizedistribution desired in the product.

The temperature of operation is dependent upon the melting point, meltflow properties, decomposition temperature, and desired fineness ofdispersion of the selected synthetic organic thermoplastic resin. Whilesuch resins can be dispersed at temperatures as low as their respectivemelting points increases in dispersion temperature beyond the meltingpoint and up to the decomposition of the resins are generallyaccompanied by corresponding increases in the fluidity of the moltenresin. As the fluidity of the melt increases, the dispersions aregenerally expected to develop lower average particle sizes withoutrequiring increases in agitation effort, although frequently very fineparticles have been produced at the lower temperatures.

In the specific embodiment of this invention for preparing polymerpowders of very fine particle size, for example, homopolymers andcopolymers of average particles size less than 10 microns, thedispersion temperature is at least 25 F above the melting point of thepolymer blend, and preferably is at least 60 F. above the melting point.

For the preferred polymers, dispersion temperatures range from about 230to 500 F. As aforementioned, the use of lower temperatures, that is downto the melting point of the polymer, will also yield dispersions, butoften of a coarser particle size.

The pressure in the dispersion vessel is adjusted to exceed the vaporpressure of the liquid medium at the operating temperature (above themelting point of the polymer blend) so as to maintain a liquid phase.More particularly, the pressures may range from about 1 to 217atmospheres, and preferably from about 6 to 120 atmospheres. In caseswhere the polymer is sensitive to air at the elevated dispersiontemperatures, an inert gas, e.g., nitrogen or helium, may be substitutedfor the air normally present.

Drying of the recovered finely divided polymer generally yields afree-flowing powder of fine particle size and relatively narrow particlesize distribution. In general, the particles of a powder so produced areall less than about 1,000 microns in diameter. By varying thecomposition of the selected dispersing agents, the ratio of polymer towater, and other material concentrations and operating conditions,powders can be made having average particle sizes ranging from about 300microns to as low as 3 microns. Especially preferred are powders ofnarrow particle size distribution and average particle size less than 20microns, and often desirably less than l microns.

For water/ polyethylene dispersions, about 1 to about 20 parts by weightof water are used per part of the resin/ coloring agent, the preferredrange being about 2.5 to about parts. The ratio of the surfactant to thepolymer/ coloring agent blend in the reactor may range from about 0.1 toabout 2 parts by weight of surfactant per part of polymer/ coloringagent with a preferred range of about 0.2 to about 1 part.

The agitator speed in the dispersion vessel used in the followingexamples is in the range of about 400 to 4,000 linear feet per minute,and the agitation time is in the range of about 1 to 24 minutes at thedispersion temperature.

By the practice of this invention there are obtained coloredthermoplastic powders composed of discrete, spherical particles in whichthe coloring agent and charge-directing agent are uniformly dispersed.The particles have diameters in the range of l to about 1,000 micronswith an average particle diameter from about 3 to about 300 micronssize. These particles are especially useful as a colored or black toneror for a photoconductive coating in electrostatic printing processes.They also will provide standard spherical particles for research; in airdissemination studies; for use as simulants for chemical, biological,radioactive, aerosol dispersion, and oceanography tracer work; and forapplication as a signal powder. They also can be used to providecolored, black, and opaque white coatings for paper and textiles, and indip-coating of heated metal parts. They can also be used to formcolored, pressed pellets where they act as porous or solid binders forother material, and they are satisfactory replacements for white finelydivided thermoplastics wherever a colored product is desired.

The nature and the amount of additive in the thermoplastic compositionaffect the physical properties of the final product.

Since many types of thermoplastics can be used and since other additivescan be incorporated in the polymer composition with the coloring agentand charge-directing agent, it is possible to produce powders withspecial physical properties as to particle size, density, color, surfacecharacteristics, aerodynamic and electrostatic properties, and so forth.

When materials, concentrations, and operating conditions normallyconducive to producing unpigmented spherical polyethylene particles wereapplied to pigmented polyethylene, only large globules or fine to coarsefibers were produced. By modifying conditions, however, particularly bylowering the temperature and by using polyethylenes of higher meltindex, powders consisting of fine spherical particles were consistentlyproduced.

When mechanically ground or irregularly shaped particles ofthermoplastic homopolymers or copolymers in the size range of about 0.25inch down to about 10 microns, some containing pigments, someunpigmented, and others containing blowing agents and other additives,were processed in the same manner except that agitation was startedsimultaneously with heating and continued throughout the heating cycle,various-sized spherical particles were produced. The size dependedprimarily upon the type of polymer blend, the amount of surfactant, andthe original size of the particle. Some, particularly the largeirregular particles, were reduced to very fine spherical particles. Theintermediate and small-sized irregular particles, i.e., below about 500microns, tended to retain their original size when spherized unlessexcessive surfactant was used, in which case reduction in particle sizetended to occur. In other cases, particularly with blowing agentadditives, finer particles tended to fuse into larger sphericalparticles to produce narrow size-distribution ranges. These results andthe hypotheses developed to explain them can serve as a basis forcontrolling average size and size distribution of the particles of thisproduct.

The mass or blend of solid polymer particles or molten polymer, asuitable surfactant, together with coloring agent, and charge-directingagent (if not previously blended with the polymer), and the dispersingliquid, usually water, are placed in the reactor. The mixture is thengradually brought to the desired operating temperature, above theinitial melting temperature of the polymer, with or without agitationwhile so doing, and then stirred at the desired speed and for thedesired time, to reduce the polymer to the desired form, size and sizedistribution of the polymer particles. The dispersion is then cooled, soas to solidify the colored polymer particles, and stabilize their shape,size and size distribution, while continuing the agitation, after whichthe particles can be separated by filtration, or centrifuging, or byotherwise removing the liquid. Rapid cooling can be obtained by ventingthe reactor to reduce the pressure, thereby volatilizing some of theliquid. The powder can then be washed and dried.

The following examples in the opinion of the inventors representpreferred embodiments of their invention:

EXAMPLE I A high melt index, low density granulated polyethylene wascompounded with 5 percent by weight of Vulcan 3 carbon black and 1percent (Allied Chemicals) Nigrosine Base N on a two-roll Thropp Millfor 20 minutes at 255 F. Two hundred and one grams of this blend wascharged with 201 grams of Pluronics Surfactant F-98 and 938 grams waterin a baffled, 2- liter, stainless-steel Parr-bomb reactor, equipped withthree, air driven, 3 inch-diameter, six blade turbine rotors. The bottom of the reactor was fitted with a l-inch ball valve and dischargeline, for quick venting and cooling of the liquid polymer particledispersion. The reactor assembly was inserted in a small electricfurnace for heating. The change was heated to 275 F. without stirring.Then, while stirring at 1,060 linear feet per minute tip speed LFPM), itwas held at 275 F. for 4 minutes. The charge was then rapidly dischargedand cooled. A fine black powder was formed. Some of the dye was founddissolved in the water phase. The powder was filtered, washed and vacuumdried for 6 hours.

A wet-screened fraction of this powder passing through a 37 mesh screenwas dried and tested for printability as a copying toner with a suitablecarrier.

Using a Xerox copying process, a very poor print was obtained. This wasunlike the powder, prepared as above, except that no dye was used, thatgave a good print. However, when using the dye-modified powder inElectrofax dry-toner copying, good prints were obtained, unlike when nodye was used. The results indicate that the Electrofax carrier induced apositive charge on the dye-modified powder surface. The small amount ofdye additive thus modified appreciably the potential toner usage for thepigmented powder. Similar results were obtained with some minorvariations in print quality when Allied Chemicals Nigrosine Spirit,concentrated powder, and concentrated crystals were used in turn as theadditive.

EXAMPLE I] When Example 1 was repeated, adding minute quantities ofsubmicron copper to the carbon-pigmented polyethylene, highlyelectrically charged or chargeable particles are formed. These showedstrong interparticle electrical attraction and repulsion not evidencedin the similarly prepared carbon-pigmented polyethylene without thesubmicron copper addition.

EXAMPLE Ill The procedure of Example I was repeated except that thecharge-directing agent was Nile Blue A. Results similar to those inExample I were obtained, when the finished product was used as a tonerin electrostatic copying machines.

EXAMPLE IV The procedure in Example I was repeated except that thecharge-directing agent was Algosol Blue. In this instance the finishedproduct when used as a toner gave acceptable prints in the Xerox processand poor prints in the Electrofax process. The results indicate thatXerox carrier induced the required negative charge on the dye modifiedpowder.

EXAMPLE V The procedure in Example I was repeated except that thecharge-directing agent was Diamine Black BH. Results similar to those inExample IV were obtained.

EXAMPLE VI Into the same equipment given in Example 1 were charged 168grams of Pluronic F-98, 914.5 grams of water and 168 grams of anethylene/ vinyl acetate copolymer having a density of 0.938 g/cc and amelt index of! and containing 5 percent by weight of Vulcan 3 carbonblack and 1 percent Nigrosine Base N. The charge was heated to 325 F andagitation at 780 LFPM applied for 6 minutes. Product recovery was thesame as in Example I. Results similar to those in Example I wereobtained, when the product was used as a toner in electrostatic copyingmachines.

Having regard to the foregoing disclosure, the following is claimed asthe inventive and patentable embodiments thereof:

1. A process for preparing colored polymer powders of controlledcharge-accepting properties and particle size and/or shape and/or sizedistribution, especially useful as toner powders in electrostaticprinting processes, comprising A. blending 1. a synthetic thermoplasticpolymer,

2. 0.001 to 1 parts by weight, per part of polymer, of a coloring agentselected from the group consisting of carbon black, phthalocyanine blue,phthalocyanine green, cadmium sulfide, cadmium sulfide selenide,

titanium dioxide, calcined iron oxide, chromic oxide and zinc oxide, and

. 0.00001 to 0.25 parts by weight, per part of polymer, of acharge-directing agent, which is a dye selected from the groupconsisting of azines, nigrosines, oxazines,

thiazines, azos, diazos, cyanines, indigosols, anthraquinones,ketonimines, xanthines, indamines and indophenols;

B. dispersing the blend in an inert dispersing liquid in the presence of0.1 to 2 parts by weight, per part of blend, of a nonionic surfactant;

C. subjecting the dispersion to agitation at a temperature above themelting temperature of the polymer, but below the decompositiontemperatures of the various constituents, while forming the polymer inparticle fon-n with the coloring agent and charge-directing agentuniformly distributed in or on the particles;

D. continuing the agitation for a time sufiicient to shape theparticles, and control their size at less than about 1,000 microns; andthen E. cooling the colored particles so as to solidify them andstabilize them in that shape and size, and size distribution.

2. A process according to claim 1 in which the polymer is particulate,and the polymer blend thus obtained is dispersed in the liquid while inparticulate form.

3. A process according to claim 2 in which the polymer is molten, andthe polymer blend thus obtained is dispersed in the liquid while molten.

4. A process according to claim 1 in which the particles are brought toa regular shape and surface configuration and average size within therange from about 1 to about 1,000 microns, with an average particlediameter from about 3 to about 300 microns.

5. A process according to claim 1 in which particle size is controlledby heating the dispersion at a temperature within the range from about230 to about 500 F 6. A process according to claim 1 in which particlesize is controlled by control of the degree of agitation within therange from about 400 to about 4,000 linear feet per minute.

7. A process according to claim 1 in which particle size is controlledby control of the amount of surfactant in the dispersion within therange from about 0. l to about 2 parts per part of polymer/coloringagent.

8. A process according to claim 1 in which particle size is controlledby maintaining the dispersion temperature at at least from about 25 toabout 60 F. above the melting point of the polymer blend.

9. A process according to claim 8 in which particle size is controlledso as to spherize the particles.

10. A process according to claim 1 in which the polymer is athermoplastic synthetic resin.

11. A process according to claim 10 in which the polymer ispolyethylene.

12. A process according to claim 10 in which the polymer is anethylene-vinyl acetate copolymer.

13. A process according to claim 1 in which the chargedirecting agent isa nigrosine dye.

14. A process according to claim 1 in which the chargedirecting agent isa Nile Blue.

15. A process according to claim 1 in which the chargedirecting agent isa Algosol Blue.

16. A process according to claim 1 in which the chargedirecting agent isa Diamine Black.

17. A process according to claim 1 in which the coloring agent is carbonblack.

18. A process according to claim 1 in which the coloring agent ispresent in an amount between 0.005 and 0.2 parts by weight, per part ofpolymer; the charge-directing agent is present in an amount between0.0001 and 0.10 parts by weight, per part of polymer; and the surfactantis present in an amount between 0.2 and 1 parts by weight, per part ofblend.

2. A process according to claim 1 in which the polymer is particulate,and the polymer blend thus obtained is dispersed in the liquid while inparticulate form.
 2. 0.001 to 1 parts by weight, per part of polymer, ofa coloring agent selected from the group consisting of carbon black,phthalocyanine blue, pHthalocyanine green, cadmium sulfide, cadmiumsulfide selenide, titanium dioxide, calcined iron oxide, chromic oxideand zinc oxide, and
 3. 0.00001 to 0.25 parts by weight, per part ofpolymer, of a charge-directing agent, which is a dye selected from thegroup consisting of azines, nigrosines, oxazines, thiazines, azos,diazos, cyanines, indigosols, anthraquinones, ketonimines, xanthines,indamines and indophenols; B. dispersing the blend in an inertdispersing liquid in the presence of 0.1 to 2 parts by weight, per partof blend, of a nonionic surfactant; C. subjecting the dispersion toagitation at a temperature above the melting temperature of the polymer,but below the decomposition temperatures of the various constituents,while forming the polymer in particle form with the coloring agent andcharge-directing agent uniformly distributed in or on the particles; D.continuing the agitation for a time sufficient to shape the particles,and control their size at less than about 1,000 microns; and then E.cooling the colored particles so as to solidify them and stabilize themin that shape and size, and size distribution.
 3. A process according toclaim 2 in which the polymer is molten, and the polymer blend thusobtained is dispersed in the liquid while molten.
 4. A process accordingto claim 1 in which the particles are brought to a regular shape andsurface configuration and average size within the range from about 1 toabout 1,000 microns, with an average particle diameter from about 3 toabout 300 microns.
 5. A process according to claim 1 in which particlesize is controlled by heating the dispersion at a temperature within therange from about 230* to about 500* F.
 6. A process according to claim 1in which particle size is controlled by control of the degree ofagitation within the range from about 400 to about 4,000 linear feet perminute.
 7. A process according to claim 1 in which particle size iscontrolled by control of the amount of surfactant in the dispersionwithin the range from about 0.1 to about 2 parts per part ofpolymer/coloring agent.
 8. A process according to claim 1 in whichparticle size is controlled by maintaining the dispersion temperature atat least from about 25* to about 60* F. above the melting point of thepolymer blend.
 9. A process according to claim 8 in which particle sizeis controlled so as to spherize the particles.
 10. A process accordingto claim 1 in which the polymer is a thermoplastic synthetic resin. 11.A process according to claim 10 in which the polymer is polyethylene.12. A process according to claim 10 in which the polymer is anethylene-vinyl acetate copolymer.
 13. A process according to claim 1 inwhich the charge-directing agent is a nigrosine dye.
 14. A processaccording to claim 1 in which the charge-directing agent is a Nile Blue.15. A process according to claim 1 in which the charge-directing agentis a Algosol Blue.
 16. A process according to claim 1 in which thecharge-directing agent is a Diamine Black.
 17. A process according toclaim 1 in which the coloring agent is carbon black.
 18. A processaccording to claim 1 in which the coloring agent is present in an amountbetween 0.005 and 0.2 parts by weight, per part of polymer; thecharge-directing agent is present in an amount between 0.0001 and 0.10parts by weight, per part of polymer; and the surfactant is present inan amount between 0.2 and 1 parts by weight, per part of blend.